Detection of human faces from within images represented by image data is currently being performed in various fields, such as authentication employing facial characteristics and facial expression recognition. Further, the positions of parts that constitute faces and characteristic points that represent the outlines of the parts are also being detected, and various methods for doing so have been proposed.
For example, Japanese Unexamined Patent Publication No. 2005-108197 discloses a method for discriminating the positions of eyes within faces included in images. In this method, first, characteristic amounts of a discrimination target image are calculated. Then, first reference data is referred to, to discriminate whether the discrimination target image includes a face. The first reference data is obtained by learning characteristic amounts of a great number of images, which are known to be of faces and in which the positions of the eyes are normalized within a predetermined degree of tolerance, and images, which are known not to be of faces. In the case that a face is detected within the discrimination target image, second reference data is referred to, to discriminate the positions of eyes within the face. The second reference data is obtained by learning characteristic amounts of a great number of images, which are known to be of faces and in which the positions of the eyes are normalized with a degree of tolerance less than the predetermined degree of tolerance, and images, which are known not to be of faces. This method enables accurate and highly robust detection of faces and eyes included therein.
Japanese Unexamined Patent Publication No. 2005-056124 discloses a method for detecting the positions of specific parts of faces. In this method, first, a facial region is detected within an image. Then, template matching with respect to four directional features is performed to calculate initial similarities of the four directional features. A periphery matching probability that represents the relationship between a specific facial part and facial parts in the periphery of the specific facial part is obtained. The position of the specific facial part is detected by a relaxation matching method employing the initial similarities and the periphery matching probability. This method enables accurate detection of the positions of exposed facial parts, even in the case that a portion of the facial parts are partially or completely concealed.
Japanese Unexamined Patent Publication No. 2005-056231 discloses a method for detecting faces and constituent parts thereof. In this method, first, the position of a face or a portion thereof is roughly detected from within an input image. The detection results are displayed by a display section, and a user inputs whether the detection results are appropriate, based on the displayed detection results. When the user input indicates that the detection results are not appropriate, finer detection is executed to detect the position of the face or the portion thereof. This method enables detection and utilization of the positions of specific faces or constituent parts thereof at an accuracy suited to the input image.
In authentication employing facial characteristics, facial expression recognition and the like, it is necessary to accurately detect characteristic points that represent the outlines of facial parts, in addition to the central positions of the facial parts that constitute faces. Such characteristic points include the inner and outer corners of eyes and central points of upper and lower eyelids that represent the outlines of the eyes, and the right and left corners of mouths and central points of upper and lower lips that represent the outlines of the mouths.
The method disclosed in Japanese Unexamined Patent Publication No. 2005-108197 is accurate and highly robust. However, the detection targets are only faces and eyes included therein. This method is not capable of detecting characteristic points that represent the outlines of facial parts that constitute faces.
The method disclosed in Japanese Unexamined Patent Publication No. 2005-056124 is capable of detecting the central positions of facial parts that constitute faces. However, this method is not capable of detecting characteristic points that represent the outlines of the facial parts. In addition, because template matching is employed in this method, if lighting conditions during photography of images, which are targets of detection, differ greatly, there is a possibility that the detection accuracy will deteriorate.
The method disclosed in Japanese Unexamined Patent Publication No. 2005-056231 is capable of detecting characteristic points that represent the outlines of facial parts. However, it is presumed that users will manually input corrections in cases that appropriate detection results are not obtained. Therefore, accuracy and robustness are not guaranteed, and a burden is placed on the users.
The present invention has been developed in view of the foregoing circumstances. It is an object of the present invention to provide a method, an apparatus, and a program for detecting facial characteristic points with high accuracy and without placing a burden on users.